Method of cutting with ceramic cutting tool having improved toughness behavior

ABSTRACT

This invention relates to a cutting tool material with improved toughness behavior. The improved toughness behavior is obtained by using carefully selected reinforcing materials. The objectives of this invention are realized by simultaneously utilizing additives resulting in different toughenings mechanisms. By the simultaneous utilization of different toughening additives, synergistic effects occur giving the tool excellent material properties. The tool material of the present invention comprises reinforcing whisker additives of one dimensional single crystals with preferably different diameter and aspect ratios, two dimensional single crystals with different thickness and aspect ratios, and, in combination therewith, phase transformation additives such as ZrO 2 .

This application is a divisional of application Ser. No. 201,931, filedJune 3, 1988, U. S. Pat. No. 4,849,381.

The present invention relates to a cutting tool material. Morespecicially, the present invention relates to a cutting tool materialexhibiting excellent toughness through the utilisation of differenttoughening additives.

A metal cutting tool is subjected to fluctuating stresses andtemperatures of dynamic nature. The conditions to which a cutting edgeis subjected in operation are severe, the temperature of the toolsurface often exceeding 1000° C. as the chip contacts the rake face ofthe tool at a pressure close of 150 MPa and at a relative speed in therange 0-1000 mmin⁻¹. Steep temperature and stress gradients also existin the cutting tool inserts. A consequence of the dynamic conditionsdescribed above is that generally several different failure mechanismsoperate simultaneously for given cutting conditions. However, for aspecific set of cutting data, one mechanism is dominating and limitingof the tool life.

One objective of the present invention is to obtain a cutting toolmaterial having a unique combination of good resistance against severalfailure mechanisms. The result is a tool material showing improved toollife and wider application area.

Known toughening mechanisms in cutting tool materials include i)transformation toughening, which utilises a phaes transformation of ZrO₂-particles dispersed in a ceramic matrix, this type of material isdescribed in U.S. Pat. No. 4,218,253; ii) whisher pull out whichutilises the bridging effect of small diameter (typically 0.06_(/) um),high aspect ratio and high strength single crystal whiskers in a ceramicmatrix as described in U.S. Pat. No. 4,543,345, and the combined effectof ZrO₂ and small diameter SiC-whisker as described in U.S. Pat. No.4,657,877.

Both of these effects have led to substantial improvements of thetoughness behaviour in certain metal cutting operations. However, thesearch for greater improvements in the properties of cutting tools iscontinuously ongoing.

The primary objective of the present invention is to provide a materialwith an improved toughness behaviour and a wider application area thanhas heretofore been known. This objective is achieved through thesimultaneous action of several toughening mechanisms. In differentpractical machining tests, it has been found that the maximum tool lifeis dependent on signle crystal size and geometry of the reinforcingadditives for a specified operation. Therefore, a particular prior artmaterial has a relative narrow optimum application area. It has nowsurprisingly been found that by adding several different classes ofreinforcing single crystals, with respect to geometry and size, not onlya widening of the application area is obtained but also an improvementin tool life is realised.

Generally, the present invention is directed to the fabrication ofwhisker-reinforced ceramic cutting tool materials characterised byincreased tool life and a wider application area that prior artmaterials. The composites comprises a matrix based on Al₂ O₃ or Si₃ N₄and up to 35 weight-% single crystals or whiskers with differentgeometries and sizes. Preferably an adequate concentration (in thealumina based matrix preferably 3-20, most preferably 5-15 weight% andin the silicon nitride based matrix up to 10 weight-%) of unstablizedand/or partially stabilized zirconia increases the toughness behaviourof the composite in a wider range of metal cutting applications thatobtainable with only one of the additives mentioned above. The aluminabased matrix may further comprise chromium in amounts corresponding to atotal of 1-20 weight-% as Cr₂ O₃. The whiskers, preferably of SiC, usedin the present invention are of a monocrystalline structure and aredivided into three types characterized by different diameters and aspect(length/diameter or e.g., diameter/thickness) ratios at least 5 weight-%of each of at least two of the three types being present in the matrix:

(i) Single crystral fibers with a diameter less than 1 _(/) um,typically 0.6 _(/) um, and an aspect ratio of 15-150;

(ii) Single crystal fibers with a diameter of 1-6 _(/) um, typically 3-4_(/) um, and an aspect ratio of 5-100; and

(iii) Single crystal discs with an equivalent diameter (hypotheticaldiameter of a circle with the same area as the disc) of 5-50 _(/) um,typically 20 _(/) um and an aspect ratio (aspect ratio being defined asratio of equivalent diameter through thickness of the disc) of 5-50,typically 10-20.

The composite may further comprise refractory nitrides or carbides toincrease hot hardness and thermal conductivity which is advantageous incertain metal cutting applications.

EXAMPLE 1

Cutting tool materials are prepared from the following startingmaterials:

A. Alumina with a grain size ≦1_(/) um;

B. SiC-single crystal with average diameter of 0.6 _(/) um and averageaspect ratio 60;

C. SiC-single crystal with average diameter of 4 _(/) um and averageaspect ratio 10;

D. SiC-single crystal with average equivalent diameter of 20_(/) um andaspect ratio 10;

E. ZrO₂ with a grain size <2_(/) um.

The SiC-single crystals are dispersed and wet milled with aluminapowder. After drying, the mixture is hot pressed at 1725° C. for 60min., and then with ZrO₂ at 1650° C. for 60 min. to 99.6 % oftheoretical density. The composition variants are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Composition weight percent                                                    Var.   A       B     C     D   E                                              ______________________________________                                        1      bal     24    0     0    0    prior art                                2      bal     24    0     0   10    prior art                                3      bal      0    24    0   10    own application                          4      bal      8    8     8   10    invention                                5      bal      8    8     8    0    invention                                6      bal     12    12    0   10    invention                                ______________________________________                                    

EXAMPLE 2

The materials from Example 1 are tested as inserts SNGN 120412 in castiron SS 0125 in an interrupted facing operation with high toughnessdemands. The following tool life ranking is obtained for differentcutting conditions (Table 2).

                  TABLE 2                                                         ______________________________________                                        Tool Life Ranking for Various Cutting Conditions                              ______________________________________                                        Cutting speed,                                                                          400      400    700    700  Total                                   mmin.sup.-1                                                                   Feed rate,                                                                              0.3      0.5    0.3    0.5  Ranking                                 mmrev.sup.-1                                                                  Variant                                                                       1         6        6      6      6    24                                      2         5        5      4      5    19                                      3         4        4      5      4    17                                      4         1        2      2      1     6                                      5         3        3      1      3    10                                      6         2        1      2      2     7                                      ______________________________________                                    

The results shows that variants 4, 5 and 6 according to the presentinvention utilise multiple toughening mechanisms and are superior to theprior art variants.

EXAMPLE 3

Inserts SNGN 120412 made according to Example 2 are tested in acontinuous turning operation of cast iron SS 0125 with high demands ofwear resistance (at a cutting speed of 700 mmin⁻¹ and a feed rate of 0.3mmrev⁻¹). The relative tool life ranking is shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Variant       1     2        3    4     5   6                                 ______________________________________                                        Relative Ranking                                                                            2     not tested  4     1   3                                   ______________________________________                                    

The results shows that improved toughening with ZrO₂ additives resultsin a reduction of wear resistance.

We claim:
 1. In a method of cutting metal wherein a cutting tool isbrought into contact with a metal workpiece and at least one of thecutting tool and metal workpiece moves relative to the other wherebymetal is removed from the workpiece, the improvement which comprisesusing as the tool a whisker reinforced ceramic cutting tool materialcharacterized by increased toughness behaviour over a wide range ofmetal cutting applications, consisting essentially of a compositedefined by a matrix of silicon nitride or alumina with up to 35 weight %whiskers homogeneously dispersed therein, said whiskers having amonocrystalline structure and said composite being characterized in thatit has at least 5 weight % of each of at least two of three whiskersingle crystals characterized by geometry and size range as follows:(i)single crystal fibers with a diameter less than 1 μm and an aspect ratioof 15-150; (ii) single crystal fibers with a diameter of 1-6 μm and anaspect ratio of 5-100; and (iii) single crystal discs with an equivalentdiameter of 5-50 μm and an aspect ratio of 5-50.
 2. In the method ofcutting metal claimed in claim 1, characterized in that the matrixconsists essentially of alumina, and zirconia in the range of 3 to 20weight % and chromium in amounts corresponding to a total of up to 20weight-% as Cr₂ O₃.
 3. In the method of cutting metal claimed in claim1, characterized in that the matrix consists essentially of siliconnitride and zirconia in an amount of less than 10 weight %.
 4. In themethod of cutting metal claimed in claim 1, characterized in that thewhisker material is silicon carbide.
 5. In the method of cutting metalclaimed in claim 2, characterized in that the whisker material issilicon carbide.
 6. In the method of cutting metal claimed in claim 3,characterized in that the whisker material is silicon carbide.
 7. In themethod of cutting metal claimed in claim 1, wherein the single crystalfibers have a diameter of about 0.6 μm and an aspect ratio of 15-150. 8.In the method of cutting metal claimed in claim 1, wherein the singlecrystal fibers have a diameter of 3-4 μm and an aspect ratio of 5-100.9. In the method of cutting metal claimed in claim 1, wherein the singlecrystal discs have an equivalent diameter of about 20 μm and an aspectratio about 10-20.
 10. In the method of cutting metal claimed in claim1, wherein the matrix consists essentially of alumina and zirconia in anamount of from 5-15 weight %.